Pulp and paper sludge (a byproduct of primary pulping operations, recycle streams or waste paper pulping and the like) represents an environmental and disposal problem for manufacturers of pulp and paper. Generally, pulp and paper sludge is unsuitable for paper making, although it generally contains the same components--cellulose, lignin, hemicellulose, calcium carbonate, clay, and other inorganic components--as those present in the paper pulp itself.
Paper sludge has traditionally been disposed of by landfilling, composting, incorporation into cement, and incineration. The latter option, in turn, creates another problem, namely, disposal of the resulting ash, which often makes up to 50% (and some times as much as 80% or higher) of the volume of the sludge itself.
The principal components of ash are calcium carbonate--in the form of precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC)--that typically constitutes 20% and up to 75% of dry sludge content, and clay. These two minerals are typically loaded into paper as a coating and filler to improve the mechanical characteristics as well as the appearance of paper. This makes papermaking sludge, particularly mixed office paper sludge, consisting of two major components, that is fiber and minerals, finely mixed with each other.
A typical recycling mill processes 600 tons of wastepaper per day, yielding 450 tons of pulp and producing 150 tons of papermaking sludge. The 228 mills currently under operation in North America produce 9 million tons of pulp residue, approximately 5 million tons of which is cellulose. The 154 European pulp and paper mills produce about 8 million tons of pulp residue, approximately 4 million tons of which is cellulose. The conversion of such waste material into glucose as alternate source of fuels, chemicals, and other useful products by acid and/or enzymatic hydrolysis of cellulose has long been desired. However, the mineral components of papermaking sludge dramatically decrease the efficiency of enzymatic hydrolysis of the cellulose component, making it technically unrealistic and economically prohibitive. Likewise, the high content of calcium carbonate in typical papermaking sludge renders the use of acid for cellulose hydrolysis practically impossible, because calcium carbonate reacts with and neutralizes the acid. For these reasons, industry has failed to develop a feasible approach to producing glucose from papermaking sludge using acid hydrolysis; and as a result, the cellulose component of papermaking sludge is generally largely or totally wasted. At best, the prior art describes utilization of paper sludge for production of low-end products of limited market value, without their chemical conversion into value-added products.
The carbohydrate content of plant materials consists of cellulose and hemicellulose, both polysaccharides. Chemical pulping of plant materials largely removes hemicellulose, which is much more susceptible to acid treatment and depolymerizes easily compared to cellulose. Hence, acid hydrolyses of conventional, chemically intact "biomass," on the one hand, and papermaking sludge, on the other, often differ substantially: the conventional and generally milder hydrolysis of biomass typically aims to produce sugars, most of which originate from hemicellulose (e.g., U.S. Pat. Nos. 2,734,836, 4,436,586, 4,511,433, 4,612,286, 4,668,340), while papermaking sludge typically contains much less hemicellulose; accordingly, the hydrolysis conditions are generally more stringent in order to break down recalcitrant cellulose into glucose.
Acid hydrolysis of cellulose and hemicellulose has been extensively studied since the beginning of this century, particularly in connection with sugar production and subsequent fermentation to ethanol; see Anderson and Porteous, "A Review of Developments in the Acid Hydrolysis of Cellulosic Wastes," IMechE, C88/85, pp. 77-86 (1985). Typical cellulosic-hemicellulosic materials for acid hydrolysis have included particulate wood in the form of chips, sawdust and shavings, and other plant materials and agricultural wastes (such as straw, bagasse, etc.), provided that the polysaccharides in those materials have appropriate chemical reactivity for acid hydrolysis. For example, Fagan, Grethlein et al. ("Kinetics of the Acid Hydrolysis of Cellulose Found in Paper Refuse," Env. Sci. Technol. 5(6):545-547 (1971)) detail processing of ball milled Kraft paper and municipal refuse (with no other components interfering with the hydrolysis) in the 180.degree. C.-240.degree. C. temperature range with sulfuric acid concentrations of 0.2%, 0.5%, and 1.0%, and on a scale of 0.5 gram with respect to paper amount. They showed that at 230.degree. C. with 1.0% acid, 52% of the cellulose could be converted to (unspecified) sugars; with 0.5% acid, the sugar yield was 39%. Subsequently, Grethlein ("The Acid Hydrolysis of Refuse," Biotechnol. Bioeng. Symp. No. 5, pp. 303-318 (1975)) disclosed that the glucose concentration in those experiments was only about 2 g/L (due to the limit of 1% slurry in the reactor). According to Mackie et al. ("Effect of Sulphur Dioxide and Sulphuric Acid on Steam Explosion of Aspenwood," J. Wood Chem. Technol. 5(3):405-425 (1985)), the action of 0.58% (w/w) sulfuric acid on aspen wood chips for 80 sec at 200.degree. C. liberated 16.4% pentosans (from the initial 18.3%), only 12% of which (i.e., 2.0% overall) was glucose. 70.5% of the initial wood was recovered from the reaction mixture as a solid residue (30.7% of that was lignin). Malester et al. ("Kinetics of Dilute Acid Hydrolysis of Cellulose Originating from Municipal Solid Wastes," Ind. Eng. Chem. Res. 31:1998-2003 (1992)) showed that for dilute-acid hydrolysis of cellulose derived from municipal solid wastes, the best conditions were the most severe ones among those tested: pH 0.34 and temperature of 240.degree. C. Under these conditions, a glucose yield of 54.3% was achieved at a reaction time of 4.6 sec. The actual acid concentrations used and the concentration of glucose obtained were not reported.
Sulfuric acid has commonly been used as an active agent, though other acids, such as hydrochloric (e.g., U.S. Pat. Nos. 4,237,110, 4,645,658, 4,650,689), phosphoric (e.g., U.S. Pat. Nos. 4,409,032, 5,486,068), nitric (e.g., U.S. Pat. Nos. 5,221,357, 5,536,325), trifluoroacetic (e.g., U.S. Pat. No. 4,400,218), and also gaseous hydrogen fluoride (U.S. Pat. No. 4,556,432), sulfur dioxide (U.S. Pat. No. 4,278,471), and sulfur trioxide (U.S. Pat. No. 4,427,584) have also been used.
Once again, materials that contain along with cellulose some other components (such as calcium carbonate in papermaking sludge) capable of effectively neutralizing an acid cannot be considered appropriate substrates for acid hydrolysis. As a result, known techniques of acid hydrolysis of cellulose (see, e.g., U.S. Pat. Nos. 3,532,594, 3,787,241, 4,160,695, 4,174,997, 4,201,596, 4,237,226, 4,242,455, 4,266,981, 4,281,063, 4,316,747, 4,316,748, 4,384,897, 4,427,453, 4,478,644, 4,529,699, 4,556,430, 4,615,742, 4,645,541, 5,486,068, 5,536,325, 5,562,777, 5,580,389, 5,597,714) have not been applied to hydrolyze cellulose in papermaking sludge.